This invention pertains to an improved process for the preparation of hindered phenolic benzylphosphonates which are useful as antioxidant stabilizers for a host of organic substrates.
The hindered phenolic benzylphosphonates are made by the reaction of a substituted 4-hydroxybenzylamine (Mannich base) with a dialkyl phosphite in the presence of a base. This process is described in detail in U.S. Pat. No. 3,790,648, which is incorporated herein by reference.
U.S. Pat. No. 3,790,648 teaches that the reaction is carried out in the melt or in an inert organic solvent such as aromatic hydrocarbons, higher-boiling ethers or aliphatic hydrocarbons. The bases used in the process are alkali amides, alkali hydrides, alkali hydroxides, alkali alcoholates or alkali dialkylphosphites.
Following the reaction of the Mannich base and dialkyl phosphite, the reaction mixture, preferably in the melt state, is neutralized with acetic acid. The crude product obtained is then recrystallized from acetone as seen in Example 3 of U.S. Pat. No. 3,790,648.
While the product obtained by the process of U.S. Pat. No. 3,790,648 is made in good yield (up to 90%) and of acceptable purity, the use of strong base with a hindered phenolic Mannich base amine sometimes leads to the formation of small adventitious amounts of color forming bodies, presumedly due to the presence of quinoid structures, which tend to discolor the phosphonate product as prepared or to develop in the product on storage.
To overcome this practical problem, a number of reducing agents were introduced into the process in an attempt to prepare phosphonate products which not only are colorless as formed, but which do not discolor on storage. Of all the reducing agents tried only oxygenated thiourea, sold as Arolite TD Concentrate by the Arol Chemical Products Company, Newark, N.J. 07105, was effective in the instant improved process. Sodium hydrosulfite, sodium borohydride and zinc-acetic acid all were relatively ineffective in preventing the discoloration in the phosphonate product whereas the oxygenated thiourea led to colorless product which did not discolor on storage.